Cell Reproduction

Chapter 6

WHY DIVIDE?

• Cell division functions in reproduction, growth and repair.

• The division of unicellular organisms reproduces the entire population.

• In multicellular organisms, division is used to repair and for growth. Also, division is needed to produce egg or sperm used for reproduction.

WHY DIVIDE?• Cells grow until they reach their size limit,

then they either stop growing or divide.• The ratio of a cell’s surface area to cell

volume determines the limit of cell growth for cells.

• Cell size is limited by the cell’s ability to transport materials and communicate instructions from the nucleus.

WATCH• Introduction to Cell Reproduction

CHROMOSOMES• Chromosomes consist of tightly packed

DNA coiled around proteins (histones) that support its structure.

• DNA contains genes which are segments of DNA that code for a protein.

• Humans have about ~20,000 genes

CHROMOSOMES

Histones

CHROMOSOMES

Eukaryotic Cells• Somatic (body) cells

– 46 chromosomes – Diploid (2n)

• Sex cells (gametes)– 23 chromosomes– Haploid (n)

CHROMOSOMES

• In Humans, there are 23 pairs of chromosomes.

• The first 22 pairs are called autosomes. • The 23rd pair, the sex chromosome differs

between males and females. • Females have two X chromosomes, and

males have one X and one Y chromosome.

VOCABULARY TO KNOW

• Chromatid – half of a chromosome• Centromere – the constriction region that

divides the chromosome into two chromatids• Sister chromatids – two identical DNA

molecules attached at the centromere• Homologous Chromosomes – a chromosome

pair, one inherited from the mother and one from the father, containing genes for the SAME trait or characteristic.

VOCABULARY TO KNOW• Chromatid, Centromere, Sister chromatids,

Homologous Chromosomes

CHROMOSOMES

• During cell division the nuclear membrane disappears and the chromatin coils up and darkens.

• This creates chromosomes that are easily visible.

• The “doubled” chromosomes are produced during DNA replication.-happens in interphase.

CHROMOSOMES• The doubled chromosomes are held

together by a centromere.

• Types of Centromeres:

CHROMOSOMES• Chromosomes can be studied to determine

genetic abnormalities• Amniocentesis: a test done on pregnant

women to get fetal cells to study.• Amniotic fluid is drawn from the womb.• Then a Karyotype is done to determine the

amount and type of chromosomes present.

KARYOTYPE• A Karyotype is the number and visual

appearance of the chromosomes in the cell nuclei of an organism or species.

• Scientists take a picture of the cells while they are dividing and blow up the picture to see the chromosomes.

• The chromosomes are cut out and sorted.• Karyotype

KARYOTYPE• Chromosomes are aligned by size,

centromere location, and banding patterns.– Autosomes first, numbered – Sex chromosomes last

• XX = female• XY = male

• Chromosome errors can be spotted such as additions, deletions, and extra or less whole chromosomes (ex. Down’s Syndrome)

KARYOTYPE

KARYOTYPE• Look at the Karyotypes given to you. • Answer the questions in Section B, #10. • Then do Section B #11-13.• Quiz Friday over page one of the notes and

vocabulary.

KARYOTYPES

CELL CYCLE• series of events of cell growth and division• all cells are on their own cycle - this is from

the time of cell division through growth to the next division.

CELL CYCLESteps of Cell Cycle:• Interphase – G1, S, G2• M phase – mitosis or meiosis• Cytokinesis

INTERPHASE• 3 parts• 90-95% of the life of the cell• G1: 1st growth phase

-cell makes new proteins• S: synthesis of DNA (DNA

replication)• G2: 2nd growth phase

-cell makes new organelles

INTERPHASEDuring interphase, a cell:• Increases in mass• Roughly doubles its organelles, enzymes,

etc.• Duplicates its DNA

For most cells, it is the longest portion of the cell cycle.

INTERPHASEG1 Phase• “G” stands for “Gap”.• Longest phase• Builds proteins, carbohydrates, and lipids

and more cytoplasm.

INTERPHASES Phase• “S” stands for “Synthesis”.• More histones are made.• DNA is replicated.

INTERPHASEG2 Phase• The cell synthesizes a variety of proteins.• More organelles are made and the

cytoplasm expands.• Mitochondria and chloroplasts divide to

make more of themselves.• Other organelles “bud” off parts.

INTERPHASE• The advantage of having three phases in

interphase is that it allows time for preparations for mitosis to happen in an orderly fashion.

• It also allows time to check that things are happening as they should.

CELL CYCLE• Sometimes the cells exit the cell cycle and

enter the G0 phase. • In the G0 phase, cells are alive and

metabolically active, but do not divide.• Many cells in the human body, including

those in heart muscle, eyes, and brain are in the G0 phase.

• If these cells are damaged, they cannot be replaced.

CELL CYCLE• Some cells can be “called back” from the Go

phase to the cell cycle by certain external cues such as growth factors.

TO DO• ANIMATION: How the Cell Cycle Works

• Do Section C

CELL DIVISIONMitosis• nucleus divides into 2 identical nuclei with

identical genetic information• In Eukaryotes

– It is the main process of growth and repair. It is the process by which unicellular and multicellular organisms asexually reproduce.

CELL DIVISION• Mitosis takes place in somatic (body) cells.• Each cell is 2N or diploid

-this means that there are 2 chromosomes of every type present.

• In humans there are 46 chromosomes.• 2 daughter cells will be formed from every parent

cell.• Mitosis ensures genetic continuity (cloning).

CELL REPRODUCTION• Organelles involved in cell division are:

-centrioles (animals only)-nucleus-spindle fibers

STEPS OF MITOSISPROPHASE• The longest phase of cell division• Chromatin condenses and

becomes visible chromosomes.• Centrioles move to opposite sides

of the cell.• Spindle fibers appear and each

chromosome attaches to one fiber.• Nuclear membrane disappears

STEPS OF MITOSISMetaphase• Fastest phase• Spindle fibers attach to centromeres.• Chromosomes line up along the equator of the

cell – midway.

STEPS OF MITOSISAnaphase• Doubled chromosomes pull apart• Sister chromatids are pulled to opposite

sides of the cell by the spindle fibers.

STEPS OF MITOSISTelophase• All the events that happened in

prophase have to be reversed. • Chromosomes decondense (uncoil)

to form chromatin• Nuclear membrane reform – there

are now two nuclei.• Spindle fibers dissolve into the

cytoplasm.

CYTOKINESIS• Cytokinesis occurs.• Cytoplasm divides.• Cell pinches in half to form two new cells.

CYTOKINESIS• Animal cells: cytokinesis results when a

fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus.

• Plant cells: the rigid wall requires that a cell plate be formed between the two daughter cells.

YIELD2 diploid daughter

cellsgenetically

identical to parent

MITOSIS IN ONION ROOT TIP

INTERPHASE PROPHASE

METAPHASE ANAPHASE TELOPHASE

TO DO• VIDEO: Mitosis and Cytokinesis • Work on Cell Reproduction Practice packet

– due next Friday.

DO NOW• VIDEO: Mitosis • COMPUTERS:

– Section E: Cell Cycle – Section F: Cell Division– Section G: Online Onion Root Tips

• PRACTICE: Work on Cell Reproduction Practice Questions.

DO NOW

• Mitosis in Onion Root Tip• Section H• Section I• Work on Cell Reproduction Practice

Questions

CELL CYCLE : CREATING GAMETES

• Three Phases: Interphase, Meiosis I and II, Cytokinesis

• Meiosis: sex cell formation• Reduces the chromosome number by half

• Meiosis

MEIOSIS

MEIOSIS• Nucleus divides twice into 4 different nuclei

with different genetic information.• In Eukaryotes, it is the formation of

gametes for sexual reproduction. • Each gamete cell will contain one of the

pairs of homologous chromosomes• As a result: 4 cells form from 1 parent cell

and they contain the haploid (N) number of chromosomes

MEIOSIS• Meiosis leads to genetic variation: mixing

of the genes within an individual’s sex cells.Ex. Tall or short Curly hair or short hair

MEIOSIS I: PMAT IProphase I • the same events that

occur in mitosis except for synapsis- the pairing of homologous chromosomes to form a tetrad

• During this time crossing over of the chromosome occurs and this leads to even more variation

MEIOSIS I: PMAT I• A single cell divides

into 2 daughter cells that are not genetically identical.

• Crossing over:greater genetic variation

MEIOSIS I: PMAT 1• The homologous

chromosomes come together and literally swap parts of themselves with each other.

• This process is called Crossing Over and ensures that the daughter cells produced after the first cytokinesis will not be genetically identical.

MEIOSIS I: PMAT 1Metaphase I• same as mitosis

MEIOSIS I: PMAT 1Anaphase I• chromosome reduction

centromeres stay in tact, chromosomes don’t separate into chromatids.

• Independent Assortment – each homologous chromosome is randomly assorted at the equator and into different gametes.

MEIOSIS I: PMAT 1Telophase I• cell splits, but the

nucleus and nucleolus don’t reform

• There is one more round of cell division

MEIOSIS II: PMAT 2Metaphase II• line up in the middle

MEIOSIS II: PMAT 2Anaphase II• chromosomes separate at their chromatids

MEIOSIS II: PMAT 2Telophase II • nucleus reforms• nucleolus reforms• membrane reforms• cytokinesis occurs• 4 haploid cells are

formed

CYTOKINESIS• division of the cytoplasm

YIELD4 haploid daughter cellsgenetically different from parent

OOGENESIS VS. SPERMATOGENESIS

• Oogenesis is formation of an egg• Only one egg is formed (three polar bodies

are also formed).

• Spermatogenesis is formation of sperm.• 4 sperm cells are formed from 1 parent cell.

OOGENESIS VS. SPERMATOGENESIS

TO DO• Animation: How Meiosis Works • Section J• Cell reproduction Practice is due Friday.

DO NOW• Get out your notes and worksheet packet.• Cell reproduction Practice is due tomorrow.• Control of the Cell Cycle

CELL CYCLE CONTROL

A Molecular Control System• Several checkpoints act as built-in stop

signs that halt the cell until they are over-ridden by go ahead signals.

• Three checkpoints exists in G1, G2, and M.• Cells also have a predetermined lifespan.

CELL CYCLE CONTROLThree checkpoints exist during interphase to make sure that everything has gone as planned and fix errors if needed.• G1-S checkpoint (end of the G1 phase) makes sure that the

DNA is intact and that the cell has enough energy to enter the S phase.

• The S phase checkpoint makes sure that DNA is replicated correctly without any breakages.

• The G2-M checkpoint at the end of the G2 phase is another safeguard in case something happens to the DNA or cell before it undergoes the massive task of dividing.

CELL CYCLE CONTROL• Timing is controlled by regulatory proteins

– cyclins and kinases. These proteins selectively access, activate and silence information in DNA.

• Contact Inhibition – Cells release chemicals to inhibit growth when they become too crowded.

CELL CYCLE CONTROL

When things go wrong?• Uncontrolled Cell Growth • Mutant genes cause tumors to form by

disrupting normal cell cycle controls• Altered cells grow and divide abnormally.• Tumors may be cancerous

TO DO• The

Effect of Contact Inhibition on Cell Division

• Mistakes in Meiosis • Cancer and the Cell Cycle • Cell reproduction Practice is due tomorrow.

DO NOW• Turn in Cell Reproduction Practice packet.• Pick up review.• Get out notes and worksheet.• How to remember Mitosis versus Meiosis• Mitosis and Meiosis

PROKARYOTIC CELL DIVISION

• Prokaryotic fission, which is binary fission, is a form of asexual reproduction and cell division used by– all prokaryotes (bacteria and

archaebacteria)– some organelles within eukaryotic

organisms (e.g., mitochondria).

PROKARYOTIC CELL DIVISION

• Most bacterial genes are located on a single bacterial chromosome (~4million base pairs) which consists of a circular DNA molecule and associated proteins.

PROKARYOTIC CELL DIVISION

• Bacteria do not have as many genes or DNA molecules as long as those in eukaryotes.

• Humans ~2.3 billion base pairs.• Bacteria circular chromosome is still highly

folded and coiled in the cell.

PROKARYOTIC CELL DIVISION

• Prokaryotes reproduce by binary fission, not mitosis.

• In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site.

• Binary Fission

PROKARYOTIC CELL DIVISION